Rhesus monkeys infected with SAIDS retrovirus type 1 (SRV-1) frequently develop a fatal immunosuppressive disease with many of the clinical and pathological features of human AIDS. HTLV-III/LAV, the causative agent of AIDS, has been found to have a narrow host cell range, replicating preferentially in T4+ lymphocytes. SRV-1 has a much broader host cell then HTLV-III/LAV, infecting such cell types as fibroblasts, kidney cells and B and T lymphocytes. Inverted T4/T8 lymphocyte ratios have not been seen in animals with SAID, regardless of their disease stage. These results suggest that SRV-1 exerts its immunosuppressive effect by a mechanism different from HTLV-III/LAV. Polymyositis was a frequent complication of rhesus monkeys who experienced fatal SAIDS. Eleven of 25 fatally infected animals showed clinical, histologic and/or histochemical evidence of polymyositis. Many of the clinical, histological and histochemical features of the polymyositis seen in these animals resembled those reported for virus-induced human polymyositis. Thus, further study of this model appears warranted. Patients with HTLV-III/LAV antibody, with or without AIDS or ARC, continue to be evaluated for possible HTLV-III/LAV related disease of the central or peripheral nervous system.